Invention patent no. ES 2 153 331 refers to an intervertebral fixation system for spinal treatments used to immobilise two or more vertebrae through the use of bars that are attached to the bones with pedicle screws.
This system allows to install the pedicle screw on a physically separate basis from the rest of the parts that make up the device, thus allowing to fix the pedicle screw in the first instance without the bar or the means of fixing the latter to the pedicle screw preventing the viewing and therefore, the handling, of the pedicle screw while it is being fixed.
This system constitutes a pedicle screw with a spherical head intended to be inserted in the bone, a tulip enabled to receive the head of the pedicle screw, containing wide notches in order to position the bar and which constitutes a strangled opening on which the head of the pedicle screw is housed. The head of the pedicle screw is held in place by a tightening screw that moves inside the tulip and has an upper indentation on which the bar is supported.
It also includes a locking cap that moves in an axial direction along with the closing screw, which threads inside the tulip and applies pressure on the bar, thus establishing the definitive fixation and tightening of the system.
On the other hand, there is also invention patent no. WO 2005/16161, which constitutes an evolution of the system described above and also incorporates on an additional basis a supporting nut which is threaded on the outer face of the tulip and on which the bar rests, so that the tightening of the supporting nut determines the height adjustment of the bar, separating the bar from the bone.
Moreover, this invention patent incorporates an alternative system for fixation of the pedicle screw that allows to place the pedicle screw in different positions as regards the bar, unlike other systems in which only the perpendicular orientation between the bar and the pedicle screw is possible. The fixation system proposed constitutes a rosette formed by an upper promontory from which a series of flexible slats separated by slots come down, defining a housing in which the spherical head of the pedicle screw is placed. The position of the pedicle screw is fixed by a tightening screw that incides on the upper face of the promontory of the rosette in order to close the slats that press against the conical wall of the tulip, thus embracing the head of the pedicle screw.
One of the problems encountered in the use of these devices refers to the fact that because the vertebrae are very close together, it is very complicated to install the tulips on the same bar and they may even come into contact with each other.
Moreover, the installation of the set formed by the closing screw and the locking cap in the tulip is not exempt from a certain degree of complication, as there are no means to fix the position of the locking cap when the assembly commences with the screwing of the closing screw, and the locking cap prevents proper viewing of the cap-screw construct while it is being assembled in the tulip, leading to a situation where the screw may even be slanted and not thread properly in the tulip.
On the other hand, in some tulips, there is the problem that the threads on the closing screws do not thread fully onto the tulip, meaning that they could break or come loose when the device is installed on the vertebrae.